Steam producing reactor and fuel therefor



March 29, 1966 Original Filed TEMPERATURE (F) HEAT FLUX (BTU/HRFT2)POSIT TEMPERATURE HEAT FLUX (BTU/HR-FT2) Oct. 11, 1962 3 Sheets-Sheet 1CLAD I""\ 3' I STREAM 0 ;;5gg,-, DEPARTURE ml.

i NUCLEA TE 50/1. NG LI/VF STEAW TRANSIT/0N OUAL TY POINT l I t 6 l I ll I l 0 .2 .4 .e 8 l0 .s .s .4 .2 o .2 .4 .e .8 l0

ION ALONG CORE LENGTH (I/L) MEASURED FROM BOTTOM CLAD x STREAM MARGINBURNOUT O .2 .4 .6 .8 L0 67 Z POSITION ALONG CORE LENGTH (l/L) MEASUREDFROM BOTTOM INVENTORS. KEITH W CAMPBELL DONALD H. /MHOFF ROBERT 7.'PENNINGTON JOHN M. ROBERTS TTORNEY.

March 1966 K. w. CAMPBELL ETAL 3,243,351

STEAM PRODUCING REACTOR AND FUEL THEREFOR Original Filed Oct. 11. 1962 5Sheets-Sheet 2 I INVENTORS.

KEITH W CAMPBELL DONALD H. IMHOFF ROBERT 7. PENN/NGTON JOHN M. ROBERTS ATTOHNE Y March 29, 1966 K. w. CAMPBELL ETAL 3,243,351

STEAM PRODUCING REACTOR AND FUEL THEREFOR Original Filed Oct. 11, 1962 3Sheets-Sheet 5 1N VEN TORS.

, KEITH W. CAMPBELL DONALD H. lMHOFF ROBERT T PENN/NGTON JOHN M. ROBERTSA TTORNEY.

United States Patent 3,243,351 STEAM PRODUCING REACTOR AND FUEL THEREFORKeith W. Campbell, Robert T. Pennington, Donald H. Irnhofi, and John M.Roberts, all of San Jose, Calif., assignors to the United States ofAmerica as represented by the United States Atomic Energy CommissionOriginal application Oct. 11, 1962, Ser. No. 230,019. Divided and thisapplication Dec. 23, 1964, Ser. No.

5 Claims. (Cl. 17678) This invention relates to neutronic reactors whichproduce steam, particularly superheated steam, within a reactor corecontainment vessel. More specifically, this invention relates tosuperheat neutronic reactors and fuel bundles therefore in which steamof varying predetermined quality is produced from water in a pluralityof passes through fuel elements. In a preferred embodiment of theinvention, superheated steam is produced.

This application is a division of our copending application S.N.230,019, filed October 11, 1962, and now abandoned.

This invention provides fuel bundles for a neutronic superheat reactorin which all of the heating and steam processing functions are performedtherein. This is accomplished by boiler rods interspersed among processtubes having sized openings and the like for pressure control. In oneembodiment the process tubes are zoned into a central superheat area fora final pass and an outer transition zone for intermediate heating. Thebundles are provided with means for an adjustable, removable pressurefit against upper and lower supporting structure, including in thepreferred embodiment a compression spring in combination with adjustablefittings as disclosed. Additional novelty resides in a central bafiieunderlying the process tube grids, steam separator means, demister andcentrifuge means, and purification means. A manifold pressure equilizeris further provided which may be used in combination with a plurality ofbundles.

A direct-cycle, boiling water nuclear reactor is one in which fissioningof the nuclear fuel produces heat which boils water contained within thecore of the reactor. The steam formed thereby may be conveyed to aturbo-generator to generate electricity. US. Patent No. 2,936,273,issued May 10, 1960, filed June 28, 1955, on behalf of Samuel Untermeyerdescribes a direct-cycle boiling water reactor which may be modified inaccordance with the teachings of this invention to generate superheatedsteam within the reactor core.

The use of saturated steam to drive a turbine is less efficient than useof superheated steam in modern conventional central station powerplants, superheated steam is invariably employed since the thermalelficiency of the turbine increases with an increase in steamtemperature. The combination of a nuclear reactor, a steam producingboiler, and a superheater into one assembly has been a primary objectiveof power reactor designers. However, heretofore, a satisfactory means ofproducing steam and superheating it within each of a plurality ofsymmetrical fuel bundles to the temperatures desired for modern powerplants has not been available.

Early proposals for superheating steam within a nuclear reactor coremade by Herbert E. Metcalf, US. patent application S.N. 649,408, filedFebruary 21, 1946, now Patent No. 2,787,593 issued April 2, 1957, andEugene P. Wigner, U.S. patent application S.N. 769,301, filed August 18,1947, now Patent No. 2,806,820 issued September 17, 1957. In theirreactor systems, however, water was heated within the reactor core andflashed into steam outside of the reactor vessel. The steam was thendirected back into the reactor core where it was superheated. The

"ice

systems described were not eflicient steam producers since steam was notinitially produced within the core.

Subsequent to the invention of the boiling water reactor by SamuelUntermeyer described in his patent application hereinbefore referencedit was proposed to boil water and superheat steam within the samereactor core as evidenced by the application of Michael Treshow, S.N.655,155, filed April 25, 1957, now Patent No. 2,938,845 issued May 31,1960. In the Treshow system, water was boiled in the reactor tankexterior to the fuel elements. The resulting steam was piped out of thereactor tank and then reintroduced into the reactor core to channelswithin the fuel elements where the steam was superheated. Considerablepiping and equipment were required to recirculate the saturated steaminto the interior of the fuel elements of the active core.

Other proposals have been made for producing and superheating steamwithin the same reactor core whereby the steam would be produced in aparticular portion of the core such as in a peripheral zone at theexterior of the core and then superheated in a central zone. In thesesystems, it has been the usual practice to produce steam as the coolantflows through a steam producing zone of the core in the oppositedirection. Proper ducting is required between the steam producing zoneand the steam superheating zone to provide for the necessary steam fiow.

An embodiment for producing superheated steam by production of saturatedsteam in the periphery of a reactor core and recirculation into thecentral active portion, disclosed and claimed in an. application ofJoseph H. Harrer et al. for U.S. Letters Patent, S.N. 27,462(60)entitled Nuclear Reactor Fuel Assembly, now Patent No. 3,049,487 issuedAugust 14, 1962, is also acknowledged. In this embodiment the outer coreregion consists of fissile fueled tubes which contact water to producesaturated steam. The saturated steam flows to a chamber at one end ofthe core and then is directed back in an in-and-out pass throughconcentric fuel tubes sealed at one end in the central core portion,whereby superheated steam for a turbine loop is produced.

While the prior art as represented by the foregoing embodiments producessaturated and/or superheated steam within the critical core region of aneutronic reactor or by contact with heated coolant therefrom, severalinherent problems in the prior art designs are apparent which precludeformation and superheating of steam in a single fuel element assembly ofa conventional reactor, particularly liquid cooled, liquid moderatedreactors employing fuel elements comprising concentric cylinders offissile materials. Specifically, the prior art embodiments produce steamin outlying or separated areas in the reactor and superheat the steam ina second pass through different portions of the reactor, therebyrequiring special construction and the solution of different engineeringproblems in different portions of the reactor Further, prior artattempts to produce superheated steam in an element submerged in liquidcoolants or moderators have not been practical because the permissibletemperature differential was limited. The usual temperature of operationin the prior art embodiments for producing superheated steam is only ofthe order of 600 F. or below, maximum. The upper temperature limitationis ordinarily determined by the materials of construction which have therequisite cross section for thermal neutrons, structural strength of thedesired temperatures, and proper sizing commensurate with costs ofconstruction and design factors. Temperatures as high as 1000 F. arethereby generally precluded and uniform temperature gradients have notbeen stressed.

A reactor embodiment in which steam is both produced and superheatedwithin each of a plurality of fuel elements with a reactor core grid isdisclosed and claimed in copending U.S. application S.N. 168,857, andnow US. Patent No. 3,121,666, entitled superheated Steam ProducingNuclear Reactor, by Clifford W. Wheelock, filed January 25, 1962, nowPatent No. 3,121,666 issued February 18, 1964. The Wheelock designprovides for in-and-out passes within the single fuel elementsincorporating, among other features, dead steam spaces adjacent thesuperheat cylinders to insulate from loss of heat to the water boilingpasses.

The present invention provides a similar function within shrouded,compact, unitized fuel bundles, with resulting advantages in fabricationand reactor control, and also incorporates additional individual novelfeatures as well as a new basic fuel bundle design. Boiler rods areprovided for producing steam during an initial pass, with water andsteam plenums at the respective bundle ends. Process tubes, either inzones divided according to heating capacity, or interspersed uniformlythroughout the bundle, provide superheating capacity. Various provisionsfor demisting, purification of water, etc., are incorporated into theembodiment. The resulting bundle is a self-contained unit for producingand superheating steam and carrying out the several other steamprocessing operations ordinarily accomplished only within the reactor asa Whole. Bundles are conveniently supported by cylinders havingadjustable spring compression means at one end of each bundle incombination with variable length support means. Precise temperaturecontrol of the superheat pass is assured by means of controlled processtube inlet means such as sized inlet openings and steam guide means suchas swirl vanes. Invention resides in the precise structural combinationand relationships of the several functional components, together withmodifications new in the reactor arts.

Unlike many prior embodiments of the art, mechanical steam separatorsand reactor recirculating piping and pumps are eliminated in thepreferred embodiment. Significantly, the structural arrangements andcombina tions enable operation of the fuel at high heat flux in theregion of the high steam quality because a minimum heat transfercoefficient is possible with steam of high quality. This minimum heattransfer coefficient permits operation at a heat flux above the normalburnout heat flux for low quality steam since it is possible to predictthe maximum fuel cladding temperatures. This results in establishing aperformance limit in the transition boiling region which is based onmaximum fuel surface temperature, rather than heat flux, in a mannersimilar to the design procedure used for establishing performance limitsof the fuel in the superheat region.

Accordingly, an object of the invention is to provide a fuel bundlecapable of producing and superheating steam therein. Another object ofthe invention is to provide a fuel bundle which is unitized, easilyremovable from a reactor core, compact, and in which is self containedall of the individual steam producing functions desired within thereactor core as a whole including boiling water, separation of moisturefrom steam, condensation of wet steam, removal of entrained Water to apoint Where purification can be undertaken, and centrifugation of wetsteam. A further object of the invention is to provide a fuel bundlehaving both boiler rods for boiling Water and process tubes forsuperheating steam mounted between water and wet steam plenums adaptedto provide controlled superheating within the individual fuel bundles. Astill further object of the invention is to provide means for steampressure control and distribution both within the individual fuelbundles and also in a manifolding system for use in combination with aplurality of bundles.

A further object of the invention is to provide such a fuel bundleenclosed in a shroud between upper and lower plenum. Another object isto provide such a fuel bundle which is adapted to be supported incompressive fit at the upper plenum exit, e.g., by spring loadedsupports held against the pressure vessel support or plenum structure. Astill further object is to provide boiler rods and/ or transitionprocess tubes mounted to produce wet steam for collection in a Wet steamplenum and superheat process tubes leading from the Wet steam plenum tothe superheat steam plenum. Another object is to provide swirl vanes andthe like within the process tubes for controlled heating, pressure anddistribution of the wet steam. Another object is to provide a centralaxial boile rod and process tube zone for superheating steam. A furtherobject is to provide a wet steam separator, demister, centrifuge unitand demineralizer outlet adjacent to or integral with a lower wet steamplenum.

The invention will be better understood upon examination of thefollowing description and figures, of which:

FIGURE 1 is a line drawing showing the relationship between heat fluxand position along the core for a single pass once-through superheatreactor;

FIGURE 2 is a line drawing showing the relationship between heat fluxand position along the core for a triple pass once-through superheatreactor;

FIGURE 3 is a cross sectional side view of a neutronic reactorembodiment incorporating the fuel bundles and bundle supports of theinvention;

FIGURE 4 is a cross sectional side view or" another preferred embodimentof the invention incorporating wet steam separation structure;

FIGURE 5 is a cross sectional end view of the fuel bundle of FIGURE 4taken along line 55 of FIG- URE 4; and

FIGURE 6 is a cross sectional side view of a neutronic reactor showingthe supporting and lower plenum structure in combination with the fuelbundle of FIGURE 4.

The once-through superheat reactor of the present invention is a thermalspectrum, water moderated boiling water, superheated steam cooledreactor where the boiling and super-heating is done in a continuous flowprocess within a single reactor vessel. Departure from nucleate boilingmust be accepted and controlled in a once-through reactor if high heatfluxes and peak performance are desired. This represents a departurefrom boiling Water reactor design practice where heat flux is limited bya burnout limit. For the once-through superheater, the fuel is protectedfrom burnout at low steam qualities, by a factor of safety in the normalmanner, but at high steam qualities in the transition region departurefrom nucleate boiling is permitted. In practice it has been found thatthe temperature rise at the point of departure from nucleate boiling issmall and reproducible.

In the heated process channel, the velocity of a boiling mixtureincreases as the steam voids increase, and at high steam qualities thevelocity is several magnitudes higher than at lower quality. In poolboiling the velocity of the blanketing steam is essentially zero, andthe principal heat transfer mechanisms at burnout are conduction throughthe vapor blanket plus radiation from the heater surface to the watersurface. Under these conditions the temperature jump or transition jumpwill be severe, resulting in burnout or melting of the heater.

With the heat transfer greatly improved by convective heat transfer in aforced fiow process channel, the transition temperature jump is lesssevere, and at high steam qualities where the vapor velocity is high,very good heat transfer coefiicients are realized. Heat transfercoefiicients of 7001200 B.t.u./hr./ft. F. are common values for steamquantities of 30 to percent. It has been found that after the transitionpoint the heat transfer coefficient changes very little until near thedry saturated range where it begins to improve again.

FIGURE 1 illustrates the characteristics for a single pass once-throughfuel element. For ease of calculation this study is based on a cosineheat flux variation with the associated change in quality from 0 topercent. Cor

responding to this quality variation is the usual burnout or departurefrom nucleate boiling limiting heat flux line. The single pass elementmust be designed with a low enough heat flux, such that the intersectionbetween the actual hot channel heat flux line and the DNB line occurs ata quality about 30 percent. It should be noted that the safety factorfor burnout at low qualities for these characteristics is unsafe. Alocal hot spot could cause burnout at low quality. A similar study for atwo pass system shows an even poorer burnout safety margin. But thethree pass system is ideal. This is illustrated in FIGURE 2. The firstpast is a conventional boiler pass exiting at an average exit quality ofpercent. Applying the usual hot channel factors and burnout safetymargin this pass becomes a conventional BWR design problem. The burnoutsafety margin improves at the end of the first pass and is quite high atthe beginning of the second pass which will henceforth be referred to asthe transition pass. By the time the actual transition point is reachedthe gap between the low quality burnout and high quality transition hasbeen safely bridged. As far as burn-out alone is concerned the threepass once-through reactor is as safe as or safer than an ordinaryboiling water reactor. It may be safer because in the once-throughreactor it is mandatory that the exit superheat temperature be closelycontrolled. This in turn sets a tight control on the exit steam qualityfrom the boiler. Thus better monitoring of the boiling phase isprovided.

The three pass system also provides a better length to diameter ratiofor the core. It can be shown that with reasonable mass velocities, heatfluxes and coolant annulus sizes, the single pass once-through reactormust be made 2030 feet long with a corresponding reduction in equivalentcore diameter. Folded three times, the length and diameter become morereasonable as in the three-pass design.

A fuel bundle 81 having provisions for moisture extraction anddemineralization between the second and third passes is shown in FIGS.4-6. The bundle is made up of three types of fuel rods or process tubesenclosed within an outer shroud 82 having water inlet ports 80. Boilingrods 84 are interspersed in the interstices of an array of annulartransition fuel elements 86 on the peripheries of the bundles togetherwith superheat fuel elements 87 on the interiors of the bundles. Bothtransition and superheat elements comprise an outer process tube 88 andan inner fueled process tube 89 forming annular coolant flow passages91.

Upper plenum base plate 92 holds boiling rods 84 by means of bores 93;while both the transition elements 86 and the superheat elements 87 aremounted communi capably through plate 92, the transition elements beingcapped by element 94 to prevent escape of steam from the transition passdirectly into superheat plenum 96. Apertures 97 in the sides of the topsof transition tubes 86 control the pressure drop. Inner process tubes 89of the transition elements 86 are further produced with swirl vanes 98,also being mounted through apertures 95 in plate 92, to centrifugemoisture to the heater surface. Vances 99 carried by transition tubes 88further control the relative steam qualities entering tubes 88 and 89 oftransition fuel elements 86.

The lower ends of rods 84 are mounted through grid 101 and the lowerends of transition and superheat process elements 86 and 87 arecommunicably mounted through lower base plate 102, fastened to neckeddown supporting cylinder 83. Centrally disposed baffie i103, octagonalin cross section, is fastened to the underside of base plate 102 andextends downwardly to provide transition and superheat zones 104 and106, corresponding in location to axial extensions of transition andsuperheat elements 86 and 87. Steam separator plate 107 spiralsdownwardly between baffle 103 and shroud 82 to which it is rigidlyattached to drain moisture to demister and centrifuge 108 (not shown)supported from the bottoin por-- tion of bundle 81. The demister 108 maybe of conventional design, such as the wire mesh packing which iscommercially available for gas demisting service.

The fuel bundle terminates below demister and centrifuge 108 and 109.Separable necked down portion 111 is joined with the core portion ofbundle 82 at coupling 112, providing easy access to the stream separator112, provided easy access to the steam separator 107, and the demisterand centrifuge 108. Fuel support column and condensate downcomer 113,into which bundles 81 are slidably mounted, are held upright bysupporting reactor bottom grid plate 114 (see FIGURE 6). Condensaterejected through the downcomers 113 enters condensate headers 116(FIGURE 6) communicating through plate 114, and is withdrawn throughline 117 to ion exchanger purification means (not shown) and/ orrecirculated through the reactor core.

In addition to reducing the fouling of the heater surfaces, thedemineralizer system provides a manifold system at the bottom of thevessel interconnecting all of the fuel bundles. This equalizes thepressure across the reactor and increases the flow in the hottestsuperheat element. This further increases the allowable reactor power.Since the coolant enthalpy is leveled to dry saturated steam across thereactor at the entrance to the superheat pass, the sensitivity of theexit steam temperature to a power change is reduced considerably overthat of a true oncethrough reactor.

Steam formed in superheat elements 87 exits through apertures 95 intosuperheat plenum 96 defined by superheat plenum grid plate 92 andhousing 121 having flanged opening defined by tubular extension 122.Slotted tubular conduit 123 is slidably mounted over extension 122 andheld from radial movement by brackets 124- fastened to extension 122through slots therein (not shown), conduit rim 126 having O-ring seals127 is held against grid 37 by compression spring 128 mounted betweenhousing 121 and rim 126. Stainless steel bellows seal 129 prevents lossof steam through the slots in conduit 123 and between conduit 123 andthe extension 122.

In the operation of a reactor incorporating fuel bundle 81 subcooledwater enters the shroud 82 through openings and fills the space betweencore rods and tubes. As the moderator contacts rods 84 it boils andexits toward the top with approximately 10 percent steam quality. Shroud82 contains the boiling moderator and provides boiling waterreactor-type void-reactivity characteristics. The wet steam passesthrough holes 97 in outer transition fuel element process tubes 88.

The wet steam mixture passes through both tube 89 and the annulusbetween tubes 89 and 88 of transition elements and exits into transitionzone 104 at a steam quality somewhere between 70 and 80 percent.Moisture is centrifuged and demisted out of the coolant. The resultingdry steam is exited into superheat zone 106 and passes through both tube89 and the annulus between tubes 88 and 89 of the superheat fuelelements 87 and finally exits into plenum 96 at the top of the bundle81.

Reactor parameters for a 300 MW(e) plant based upon the foregoing designand disclosed in full in General Electric Report GEAR-3633, entitledEconomic Study for 300 MW(e) Once-Through Superheat Reactor, availablefrom T.I.S. Department of Commerce, Washington 25, D.C., are as follows:

Summary of once-through reactor data A. Thermal power 833.0 MWT.

B. Steam conditions:

1. Exit steam condition 980 p.s.i.a 900 F. 2. Inlet condition 1194p.s.i.a. 400 F. 3. Steam flow 2,616,000 lbs/hr.

trol elements (1. Cross sectional dimensions e. Effective length f. Typeof drive 9.36 x 9.36". 11.5 ft.

Hydraulic locking piston.

16. Peak/average burn up ratio) 7 8 C. Reactor description: D.Performance Data:

1. Reactor vessel- 1. Reactor coolant Water and/or steam.

21. Inside diameter 12.5 ft. 2. Reactor mass flow rates b. Height 49.50ft. (lbs./hr.-ft.

c, Wall thicknes 6.25 inches. a. Boiler pass: (1. Material A" $5. clad.Ave. "091x e. Design pressure 1450 p.s.i.a. Max. 1135x10 f. Designtemperab. Transition pass:

ture 650 F. Ave. 565x10 2. Reactor core 10 Max. .707 10 a. Activeequivalent c. Superheat pass:

diameter Ave. 1.0 10 b, Active height 11.5 ft. Max. 125x10 c. Activecore load- 3. React or coolant outlet ing 73 fttemperature 900 F. d.Tot-a1 uranium 4. Steam pressure loading 42,709 g- (p.s.i.a) 980 exit.e. Overall average 5. Steam coolant flow 2,616,000 lbs/hr.

enrichment 6. Max imum fuel temp.

of core 2.90 a. percent. (125% power) c 3800 F. Final enrich- 7. Maximumclad tempere t 1.31 a. percent. ature (100% power 1250 F. Plutonium at8. Maximum heat flux end of life 0.68 a. percent. 3 P1 400 000 B /h f 2p p a. 0i er pass s .t.u. r.- t. f. tg f i f l j f fi Light Water. b.Transition pass 432,000 B.t.u./l1r.-ft. g. Moderator to fuel 9 Asuperheat P 540,000 -u./hr.-ft.

tio 24a verage heat flux for Ta lI'8Cl3OI Refiectcrf h an 21. Boilerpass 117,200 B.t.u./hr.-ft. Ma tena1 Llg t water b. Transition pass120,000 B.t.u./hr.-ft. Axlfil thlFkness 8 c. Superheat pass 150,000B.t.u./hr.-ft. Radlal thlskness 10. Average core power On equivalentdensity 39.7 kw./liter. dia 13 inches- 11. Average specific 4. Fuelelements-Boiler power 19.5 kW./kg.

rods 12. Peak to average powa. Fuel material U0 er ratiosb. Fuel elementge- Boiler! ometry Solid rod. 40 Axififl c. Clad material Zircaloy.Radlal d Fuel dianb Hot spot factor 1.2.

etel. 035 inch Peak to average c. Clad thickness 0.026 inch. C h a rFuel clad gap m1 Overpower 1.25. (cold) Total 3.40. Gap material b.Transition super- 5. Fuel elements-Transiheat.

tion-superheat 7 Puel material Qiifiltjjjjijj. 1135: Fual element Hotspot factor 1.25.

tometry: Peak to aver-age Annular I.D. 0.320. cross channel 1.22. c.Clad material 304 SS. overpower 125 (1. Process tube m a- T t l 3.60

terial Zir l y- 13. Average core flow vee. Process itube dilo iiti (ft/sec.)- ion; a. Boiler pass:

I.D. 0.844". Inl t 0.1). 0.904". Exit f. Lattice spacing 1.264 inch.Transition P g. Shroud dimension 8.6" x 8.6"X .030". lfllfet h. Shroudmaterial Zircaloy. Exit 7. Reactor Control sulpelrheat pass 111 0 a.Method of control Control rod movement. f

b. Absorber material Boron steel.

c Number of com 14.Fuel management 20% batch reloading.

' 15. Average fuel burnup 18,750 mWd./mtu.

While the invention has been disclosed with respect to several preferredembodiments, it will be apparent to those skilled in the art thatnumerous variations and modifications may be made within the spirit andscope of the invention and thus it is not intended to limit theinvention except as defined in the following claims.

What is claimed is:

1. In a bundle of fuel elements for a water boiler superheat reactor,the combination comprising:

a plurality of boiler rods interspersed between a plurality of processtubes, each carrying an inner concentric fuel tube;

means for supporting said rods and said tubes in parallel spacedalignment;

a shroud encasing said boiler rods and process tubes joined with saidmeans for supporting same, said shroud having openings at one end foradmission of reactor coolant;

a superheat plenum receiving the open ends of at least a portion of saidtubes and adapted to receive superheated steam therefrom;

a plenum having an outlet therefrom;

a mixing plenum receiving the open ends of at least a portion of saidprocess tubes and the distal ends thereof from the superheat plenumadapted to receive low quality Wet steam therefrom;

superheat plenum outlet structure detachably joined with said superheatplenum and adapted for a compression fit with core plenum structure;

means for rigidly mounting said fuel bundle within a reactor core; and

a compression spring radially encircling one end of said bundle withstops against said bundle structure and against structure on one end ofsaid bundle adapted to yield under pressure.

2. The fuel bundle of claim 1 in which sized apertures in the upperportions of the process tubes are provided to facilitate pressurecontrol.

3. In a bundle of fuel elements for a water boiler superheat reactor,the combination comprising:

a plurality of boiler rods interspersed between a plurality of processtubes each carrying an inner concentric fuel tube, said boiler rodsterminating at the lower ends above the ends of the process tubes, saidprocess tubes having sized openings at the top ends thereof tofacilitate pressure regulation and steam distribution;

grid means at the lower ends of the boiler rods supporting said boilerrods and having openings therein for passage of moderator fluid;

upper superheat plenum structure having an upper outlet adapted torigidly hold the upper ends of said boiler rods and process tubes andhaving openings in the lower portion thereof communicating with saidinner concentric fuel tubes;

lower processing plenum structure adapted to rigidly support the lowerends of said process tubes, said upper plenum wall having openingscommunicating with said process tubes, said lower processing plenumcontaining means for separating moisture from wet steam; and

means for removing same from said fuel bundle for purification.

4. The fuel bundle of claim 3 in which swirl vanes are carried by saidprocess tubes furtherest from the longitudinal axis of the bundle in theannular spaces between said process tubes and said inner concentric fueltubes.

5. In a bundle of fuel elements for a water boiler superheat reactor,the combination comprising:

a plurality of boiler rods interspersed between a plurality of processtubes each carrying an inner concentric fuel tube, said boiler rodsterminating at the lower end of the bundle above the ends of the processtubes, said process tubes having means to regulate pressure and amountof steam admitted to the upper end thereof;

grid means at the lower ends of the boiler rods supporting said boilerrods and having openings therein for the passage of moderator fluid;

upper superheat plenum structure having upper outlet, said plenumstructure being adapted to support said tubes without passage of lowtemperature moist steam into said superheat plenum from the bundle, saidsuperheat plenum being further adapted to support said inner concentricfueled process tubes with openings for transmission of superheated steamfrom said inner concentric tubes to said superheat plenum;

a lower process plenum rigidly retaining the lower ends of said processtubes in matching openings therethrough;

a shroud enveloping said bundle and sealed against said upper and lowerplenum structure, said shroud having openings for introduction ofmoderator fluid thereto;

a cylindrical support column slida-bly retaining the lower portion ofsaid fuel bundle including stop means in combination therewith limitingthe length of the slip -fit, and adjustable compression holddown springmeans encircling said superheat plenum outlet in combination withoverlapping extension of said superheat outlet, said spring means beingadapted to force said outlet extension upwardly away from said outlet;and

means for forming a compression fit between said superheat steam plenumoutlet and a reactor superheat steam receiving means.

References Cited by the Examiner UNITED STATES PATENTS 3,049,487 8/1962Harres et al l7654 3,121,666 2/1964 Wheelock 176-54 3,185,630 5/1965Ammon 17654 LEON D. ROSDOL, Primary Examiner.

MELVIN I. SCOLNICK, Assistant Examiner.

1. IN A BUNDLE OF FUEL ELEMENTS FOR A WATER BOILER SUPERHEAT REACTOR,THE COMBINATION COMPRISING: A PLURALITY OF BOILER RODS INTERSPERSEDBETWEEN A PLURALITY OF PROCESS TUBES, EACH CARRYING AN INNER CONCENTRICFUEL TUBE: MEANS FOR SUPPORTING SAID RODS AND SIAD TUBES IN PARALLELSPACED ALIGNMENT; A SHROUD ENCASING SAID BOILER RODS AND PROCESS TUBESJOINED WITH SAID MEANS FOR SUPPORTING SAME, SAID SHROUD HAVING OPENINGSAT ONE END FOR ADMISSION OF REACTOR COOLANT; A SUPERHEAT PLENUMRECEIVING THE OPEN ENDS OF AT LEAST A PORTION OF SAID TUBES AND ADAPTEDTO RECEIVE SUPERHEATED STEAM THEREFROM; A PLENUM HAVING AN OUTLETTHEREFROM; A MIXING PLENUM RECEIVING THE OPEN ENDS OF AT LEAST A PORTIONOF SAID PROCESS TUBES AND THE DISTAL ENDS